Ok i did it u must modify
OrderInvoice class is located in /classes/order/OrderInvoice.php
put this code in function getProducts after $row['id_address_delivery'] = $order->id_address_delivery;
$sql = "SELECT `name` FROM `ps_category_lang`, `ps_product` WHERE `ps_product`.`id_product`='".(int)$row['id_product']."' AND `ps_product`.`id_category_default`=`ps_category_lang`.`id_category` AND `ps_category_lang`.`id_lang`=2";
$row['product_category'] = Db::getInstance()->getValue($sql);
Ive got only 1 language shop so u could use diff id_lang.
and u can happy use
{$order_detail.product_category}
in invoice.tpl